The invention relates to a method and an assembly for machining thin sheets and thin-walled, single or two-fold curved, three-dimensionally shaped sheets, plates, or shells, in particular by material-removing machining methods, such as milling and drilling.
With material-removing machining of thin-walled sheets, the machining forces and also the feed rate depend significantly on the possibilities of fixing the part during cutting by the connection to the remaining grid. This is true especially when making small parts, where often the typical vacuum tension is not applied across a suitably large surface in order to hold the part.
As is known, it was attempted to achieve a greater static friction caused by the vacuum by increasing the surface adhesion factor of exchangeable substratum made of paper or similar materials that are partly air-permeable. Adhesive sprays or similar means, however, are difficult to manipulate in a production-technical sense, since at the same time, all unwanted particles, such as shavings and the like, adhere to the contact surfaces, representing substantial risk for an acceptable vacuum tension.
Known substratum with a suction cup character, such as rubber, are cost-intensive and not well suited for small parts.
DE 201 17 390 U1 discloses a device for clamping plates to be machined with vacuum pressure. The device has a flat plate, which has through-holes leading from its upper side to its lower side. Between the upper side of the support plate and the lower side of a workpiece plate to be machined, a defined, air-permeable layer of filter paper is arranged. Via a vacuum pump, a partial vacuum can be produced, which acts on the workpiece via flow-through throttle means and via the defined, air-permeable layer. In this manner, it is possible to produce a sufficiently large vacuum pressure for clamping, independent of the respective contour of the plate to be machined on its lower side.
In a further development, the support plate is not dimensionally stable, rather is thin and flexible, in the manner of a film. When such a support plate is arranged with interposition of air-permeable filter material of defined thickness on the base plate, a defined upper side of the support plate is provided, which is suited completely for clamping of plates to be machined with interposition of the defined, air-permeable material.
It is disadvantageous to use two, thin-walled, film-like layers, which serve for the defined production of a vacuum. Thus, by means of sliding friction on the defining layers of these two films, local displacement, and therewith, position changes of the workpiece to be machined, can occur. In addition, the technological manufacturing preparation requires the mounting of two flexible films on the vacuum table.
A further disadvantage is that especially with heavy structured workpieces, for example skeletal or fishbone workpieces, and with machining with high cutting and feed rates, such as, for example, those with milling with end mills, a safe positional fixing of the workpiece on the vacuum table cannot be realized.
DE 40 30 113 C2 discloses a device for clamping plates to be machined, in which the base plate, on which the workpiece to be machined is accommodated, has a plurality of densely spaced through-bores with a diameter between 0.1 and 1.0 mm. A disadvantage of this assembly is the relatively high manufacturing costs for making the base plate.
In order to preclude contact between the main blade of the rotating cutting tool and the surface of the base place during machining, especially with milling, in a preferred further development an exchangeable abrasive material is placed on the upper side of the base plate. The filter paper that preferably is used effects a change of the volume flow upon application of a vacuum pressure on the base plate. The object of the filter paper is additionally to retain the cuttings falling during the machining of the plate before penetration into the bores of the base plate, in order to prevent damage to the vacuum pump.
DE 87 03 223 U1 discloses a vacuum clamping plate, in which on the surface of the clamping surface, an adhesive coating is located, which preferably comprises a perforated mat made of rubber or plastic, or alternatively, is sprayed onto the clamping surface. The reusable adhesive coating, therefore, should guarantee the absorption of the acting feed forces with roller or front milling. With the proposed use of an elastic adhesive coating, this object cannot be solved technically. Thus, with the use of an adhesive coating made of perforated rubber or perforated plastic, under the effect of the acting cutting and feed forces a torsion-like movement of the clamped workpiece results, which leads inevitably to position or form abnormalities.
The object of the invention is to eliminate the disadvantages of the prior art and to propose a method for the secure accommodation of clamped workpieces to be machined, especially of heavily structured, perforated parts on a vacuum table, as well as an assembly for performing the method.